1. Field of the Invention
The present invention relates to a highly pure monoalkylphosphine which is useful as a starting material for use in epitaxial growth by a Metal Organic Chemical Vapor Deposition (MOCVD) method and the like, and a method for producing same. More specifically, the present invention relates to a highly pure monoalkylphosphine which is substantially free of sulfur or silica, and a method for producing same.
2. Description of the Related Art
Recently, compound semiconductors have been widely used in various fields such as light emitting diodes, semiconductor lasers, and high electron mobility transistors (HEMT). An epitaxial crystal-growth technique, such as a Metal Organic Chemical Vapor Deposition (MOCVD) method, has often been used as a method for preparing compound semiconductors. Compound semiconductors produced by such an epitaxial crystal-growth technique include Group III-V compound semiconductors, and phosphine which contains phosphorus atom is used as a source of Group V elements.
However, phosphine has a safety problem since it is highly toxic and is in a gaseous form at ordinary temperatures. Accordingly, when phosphine is used for producing a compound semiconductor, it is necessary to use it under high pressure.
Recently, use of monoalkylphosphine in place of conventional phosphine has been proposed in order to avoid the dangers associated with phosphine. Although it is not desirable for the epitaxial growth film to contain carbon as an impurity, monoalkylphosphine allows only a small amount of carbon to be mixed in the epitaxial growth film, and it has a lower toxicity than phosphine; thus, monoalkylphosphine has been catching attention as a substitute for phosphine.
As methods for preparing monoalkylphosphines, there have been known, for instance, those comprising reducing phosphonium chloride and phosphonous acid, such as described in Z. anorg. allg. Chem. 433, 42 (1978) and the like. Japanese Patent Laid-Open Nos.4-9392 and 4-9391 disclose methods in which phosphonium chloride is produced by the Grignard method or the Friedel-Crafts method respectively, and is reduced by the use of a reducing agent such as lithium aluminium hydride to give a monoalkylphosphine.
In these methods, it is very difficult to obtain a highly pure monoalkylphosphine since the metallic reducing agents used for the reduction reaction, a magnesium compound in the Grignard method and an aluminium compound in the Friedel-Crafts method, tend to become included as metal impurities. In addition, the production processes are multistage processes which result in decreased yield and disadvantages in the practice thereof on an industrial scale.
In the specification of U.S. Pat. No. 5,354,918, it is described that an alkanesulfonic acid, such as methanesulfonic acid, is used as a catalyst to produce phosphine and olefin. In this patent, the methanesulfonic acid catalyst used is mixed in an organic solvent containing monoalkylphosphine. Thus a step to remove the catalyst, comprising, for example, washing with an alkali aqueous solution such as sodium hydroxide aqueous solution, becomes necessary. Nevertheless, even after such procedures are carried out repeatedly, a trace amount of methanesulfonic acid remains and it is difficult to remove the methanesulfonic acid completely by subsequent purification processes such as distillation and precision distillation.
If a trace amount of sulfur or silica is mixed in the monoalkylphosphine, the carrier concentration of the compound semiconductor formed from crystals prepared through the epitaxial growth of the monoalkylphosphine is lowered, and the product cannot be used for applications requiring a high purity, such as an embedded laser.